The present invention relates generally to multiple gain region systems, and more specifically to VCSEL systems with transverse P/N junction.
Vertical cavity surface emitting lasers (VCSELs) are revolutionizing the field of telecommunications. They generally consist of a pair of semiconductor mirrors defining a resonant cavity containing a gain medium of semiconductor materials for amplifying light.
VCSELs are generally characterized by a pair of mirrors, generally referred to as distributed Bragg reflectors (DBRs), between which an optical cavity is located. The entire structure can be formed over a substrate wafer by a process called organometallic vapor phase epitaxy (OMVPE), sometimes referred to as metal organic chemical vapor deposition (MOCVD). The optical cavity generally also includes spacer layers and an active region. The active region typically includes one or more quantum wells. The quantum wells, which typically include a quantum well layer sandwiched by a pair of adjacent barrier layers, are the layers into which carriers, i.e., electrons and holes, are injected. The electrons and holes recombine in the quantum well and emit light at a wavelength determined by the material layers in the quantum well. The quantum well layer typically comprises a low bandgap semiconductor material, while the barrier layers typically have a bandgap higher than the bandgap of the quantum well layers. In this manner, when the device is subject to forward bias, electrons and holes are injected into and trapped in the quantum well layer and recombine to emit coherent light at a particular wavelength.
Electrically-pumped VCSELs include a P-I-N junction structure in which the intrinsic material that forms a quantum well is sandwiched between layers of p-type and n-type material. For optimal light emission, the quantum well should be located at a peak of the standing wave (the axial or longitudinal mode) generated in the optical cavity. However, in a VCSEL having multiple quantum well active regions, each set of the quantum wells is separated by fifty to several hundred nanometers.
The number of quantum wells in a VCSEL determines the optical gain of the VCSEL. Before coherent light is emitted from a VCSEL, many losses inherent in the VCSEL must be overcome to reach lasing threshold. Losses in a VCSEL are created by the mirrors, diffraction, and distributed losses. Losses in the mirrors are due to the mirror reflectivity being less than 100%. Indeed, if the mirrors were 100% reflective, light could not be emitted from the VCSEL. Diffraction losses are caused when the emitted light expands as it propagates away from a guiding aperture in the VCSEL. Distributed losses are caused by scattering and by absorption of light in the VCSEL structure. The QW gain must be sufficient to overcome these combined losses. Therefore multiple QW active regions are advantageous, especially for high temperature operation and for enabling modulation at high data rates.
If more QWs are required than can fit under one maximum of the standing wave that exists in the cavity, additional QW active regions may be included in a resonant periodic gain (RPG) arrangement. An RPG VCSEL structure includes two or more active regions, with each active region lying under separate maxima in the standing wave (thus separated by some multiple of ½ wavelength). This RPG structure is used to maximize the optical gain of VCSELs.
While RPG VCSELs have been demonstrated in the more common GaAs- and InP-based materials used for red and near-infrared emitters, GaN-based VCSELs are far more challenging. This is due to their relatively poor optoelectronic characteristics, including less efficient carrier transport, defective structure, etc., which translate into low gain and high loss. Thus, for GaN-based ultraviolet or visible VCSELs exhibiting high efficiency and low threshold current and voltage, an active region incorporating several sets of QWs in an RPG arrangement is especially beneficial for achieving the requisite optical gain.
However, uniform electrical pumping of multiple GaN-based active regions has not been demonstrated. Moreover, pumping multiple GaN-based active regions with a single p-n junction is very challenging, due in part to the poor hole-transport in GaN.